[Rd] Questions on the R C API

[Gabriel Becker]

Hi Martin and Jiefei,

A quick note, I'll try to respond more completely later.


> It is a primitive but I don't understand how it manage to do that.
>

Primitives and Internals are allowed to be, but usually aren't, SPECIALSXPs
rather than BUILTINSXPs. SPECIALSXPs receive their arguments unevaluated
when called from R. See
https://cran.r-project.org/doc/manuals/r-release/R-ints.html#g_t_002eInternal-vs-_002ePrimitive.
This is not possible for functions that are not part of the R sources as
far as I know.

Also the difference between Primitives and internals isn't evaluation of
arguments (thats special vs builtin) but rather, whether they behave like a
closure (internal) or not (primitive).

Evaluation behavior for internal and primitive functions is defined within
the function table in names.c, the comment there explains the format.

Best,
~G

>
> Best,
> Morgan
>
>
>
> > Best,
> > Jiefei
> >
> >
> > On Mon, Nov 4, 2019 at 2:41 PM Morgan Morgan <morgan.emailbox using gmail.com>
> > wrote:
> >
> >> Hi All,
> >>
> >> I have some questions regarding the R C API.
> >>
> >> Let's assume I have a function which is defined as follows:
> >>
> >> R file:
> >>
> >> myfunc <- function(a, b, ...) .External(Cfun, a, b, ...)
> >>
> >> C file:
> >>
> >> SEXP Cfun(SEXP args) {
> >>   args = CDR(args);
> >>   SEXP a = CAR(args); args = CDR(args);
> >>   SEXP b = CAR(args); args = CDR(args);
> >>   /* continue to do something with remaining arguments in "..." using
> the
> >> same logic as above*/
> >>
> >>   return R_NilValue;
> >> }
> >>
> >> 1/ Let's suppose that in my c function I change the value of a inside
> the
> >> function but I want to reset it to what it was when I did SEXP a =
> >> CAR(args); . How can I do that?
> >>
> >> 2/Is there a method to set "args" at a specific position so I can
> access a
> >> specific value of my choice? If yes, do you have an simple example?
> >>
> >> 3/ Let's suppose now, I call the function in R. Is there a way to avoid
> >> the
> >> function to evaluate its arguments before going to the C call? Do I have
> >> to
> >> do it at the R level or can it be done at the C level?
> >>
> >> Thank you very much in advance.
> >> Best regards
> >> Morgan
> >>
> >>         [[alternative HTML version deleted]]
> >>
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> >>
> >
>
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>
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